In the lithographic technology, lithographic exposure tools for transferring a fine circuit pattern onto a wafer to produce an integrated circuit have hitherto been extensively used. With the trend toward a high degree of integration, a high speed and a high function in an integrated circuit, the integrated circuits are becoming finer, and the exposure tools are required to have a large focal depth and form a high-resolution circuit pattern image on a wafer surface. The wavelengths of exposure light sources are becoming shorter. ArF excimer lasers (wavelength: 193 nm) have come to be used as exposure light sources in place of the g-line (wavelength: 436 nm), i-line (wavelength: 365 nm) and KrF excimer lasers (wavelength: 248 nm) heretofore in use. Furthermore, use of an F2 laser (wavelength: 157 nm) as an exposure light source for conforming to next-generation integrated circuits having a line width of not more than 100 nm is thought to be promising. However, the generations which can be covered by this light source are regarded as being limited to ones with line widths down to 70 nm.
Under such technological trends, a lithographic technique employing EUV light as a next-generation exposure light source is thought to be applicable to plural generations of 45 nm and finer and is attracting attention. The EUV light as referred to herein refers to light having a wavelength band in the soft X-ray region or vacuum ultraviolet region. Specifically, it refers to light having a wavelength of from about 0.2 to 100 nm. At present, use of a lithographic light source of 13.5 nm is being investigated. The exposure principal in this EUV lithography (hereinafter abbreviated as “EUVL”) is equal to that in the conventional lithography in the point that a mask pattern is transferred with an optical projection system. However, since there is no material which transmits light in the EUV light energy region, a refractive optical system cannot be used, and a reflective optical system should be used (see Patent Document 1).
The reflective type mask for use in EUVL is basically constituted of (1) a glass substrate, (2) a reflecting multilayered film formed on the glass substrate and (3) an absorber layer formed on the reflecting multilayered film. As the reflecting multilayered film, a film having a structure formed by periodically stacking, in an nm-order thickness, materials having a different refractive index at the wavelength of the exposure light from each other is used. Known typical materials are Mo and Si.
Furthermore, Ta and Cr are being investigated for the absorber layer. The glass substrate is required to be made of a material having a low coefficient of thermal expansion so as not to be distorted even upon irradiation with EUV light. Use of a glass having a low coefficient of thermal expansion or a crystallized glass having a low coefficient of thermal expansion is being investigated. In this description, a glass having a low coefficient of thermal expansion and a crystallized glass having a low coefficient of thermal expansion are hereinafter referred to inclusively as “low-expansion glass” or “ultralow-expansion glass”.
The low-expansion glass or ultralow-expansion glass most widely used in EUVL reflective type masks is quartz glass which comprises SiO2 as a main component and to which TiO2, SnO2 or ZrO2 is added as a dopant for the purpose of reducing a coefficient of thermal expansion of glass.
A glass substrate is produced by processing such a glass or crystallized glass material with high accuracy and cleaning it. In the case of processing a glass substrate, in general, a surface of the glass substrate is pre-polished at a relatively high processing rate until the glass substrate surface has given flatness and RMS in a high spatial frequency (HSFR) region; and thereafter, the glass substrate surface is finished by a method having higher processing accuracy or under processing conditions bringing about higher processing accuracy so as to result in desired flatness and RMS in an HSFR region.
Patent Document 2 discloses that the polishing method and device disclosed therein are suitable for polishing processing of an optical element with high accuracy comprising a fluoride based crystal material such as calcium fluoride, magnesium fluoride, etc., which is suitable for various optical elements used over a wide wavelength range of from a vacuum ultraviolet region to a far-infrared region, lenses, window materials, prisms, etc. Furthermore, Patent Document 3 discloses that the production method of a glass substrate for use in mask blanks disclosed therein reduces or eliminates adverse influences by striae of the glass substrate for use in mask blanks or by reflection on the back surface, measures the irregular shape on the surface of the glass substrate to be measured with high accuracy, and controls the flatness with extremely high accuracy based on the measurement results, thereby realizing a high flatness.
The polishing method and device described in Patent Document 2 are based on the assumption that in the case of works comprising a crystal material are uniformly polished at a constant rate and a constant pressure utilizing a tool which is sufficiently small relative to the works, the removal amount is equal. However, the polishing tool disclosed in this patent document is one prepared by laminating a circular pitch or foamed polyurethane, as a polishing pad, onto a base metal; in the polishing method described in this patent document, such a polishing tool is pressed against the surface to be processed while being rotated and while applying a polishing liquid containing diamond fine powder thereto and continuously moved and scanned from end to end on the lens surface. Therefore, there is a concern that even when polishing is uniformly effected at a constant rate and a constant pressure, the polishing amount does not become constant depending upon the abrasion and clogging of the polishing pad and the concentration and entrance of the diamond slurry to the polishing pad.
In the method disclosed in Patent Document 3, it is necessary to make the distance L1 between surfaces A and B and the distance L2 between surfaces C and D relatively large in the surface shape measurement device 2 shown in FIG. 2 of this patent document. Specifically, it is necessary that the distances L1 and L2 are made to be about several tens mm. In measuring the surface shape, it is liable to be influenced by air fluctuation of this space. In particular, when a downflow is applied for the purpose of increasing the degree of cleanness within the surface shape measurement processing device, the influences of air fluctuation become remarkable.
Patent Document 1: JP-T-2003-505891
Patent Document 2: JP-A-2003-159636
Patent Document 3: JP-A-2006-133629